A polarization-maintaining optical fiber is given a difference between propagation constants of two polarization modes that have linear polarization along two orthogonal principal axes in the core by making the modes of the fiber not be degenerated by applying anisotropy of the stress distribution in a single-mode optical fiber. Since the structure enables the distinction between the two polarization modes, when light that agrees with a particular polarization mode is launched to the optical fiber, the light propagates through the optical fiber maintaining only that polarization mode.
As a typical polarization-maintaining optical fiber, a PANDA fiber is known. The PANDA fiber, however, requires high technology that bores holes through two places in a base material (cladding) of the optical fiber in extremely close proximity to the core region in the fabrication process, and that fills the holes with a stress-applying material to form the fiber. In particular, the process of squeezing the stress-applying material into the base material is a major factor of reducing the productivity of the polarization-maintaining optical fiber. For this reason, the PANDA fiber usually costs 100 or more times higher than an ordinary single-mode fiber. In addition, since the propagation constant difference between the two orthogonal polarization modes resulting from the PANDA fiber structure is rather small, it is difficult to reduce the crosstalk between the two modes to less than −30 dB.
Thus, it is difficult for the PANDA fiber to transmit a signal pulse train over a long distance maintaining the single polarization. Accordingly, it is not used as a single polarization transmission path. Considering such difficulties in fabricating the PANDA fiber, optical fibers with claddings with a variety of structures have been developed today.
FIG. 1 is a cross-sectional view showing a structure of a conventional polarization-maintaining optical fiber based on a photonic crystal structure. The polarization-maintaining optical fiber comprises a core region 41, a photonic crystal cladding 42 and a jacket 43. In FIG. 1, the photonic crystal cladding 42 is divided into four segments 42a, 42b, 42c and 42d by broken lines from the center to the periphery.
In the segments 42a, 42b, 42c and 42d, the grating constant Λ of the diffraction grating that consists of grating holes indicated by circles in FIG. 1 is the same throughout the grating. However, the diameter d2 of individual grating holes in the first opposed segments 42a and 42c is greater than the diameter d1 of individual grating holes in the second opposed segments 42b and 42d adjacent to the first opposed segment (d2>d1). Such a structure can bring about the propagation constant difference between x and y directions, thereby being able to implement the polarization maintaining property.
FIG. 2 is a graph illustrating variations in the modal birefringence when varying the ratio of the diameters of the air holes of the polarization-maintaining optical fiber. The detail of the calculation is described in “Polarization maintaining holely optical fiber” (Kawanishi and Okamoto, 2000 Communications Society Conference No. B-10-153 of The Institute of Electronics, Information and Communication Engineers of Japan).
The modal birefringence B is given by the following expression when the propagation constants corresponding to the two perpendicular polarization modes (HE11x mode and HE11y mode) in the fiber are βx and βy.B=(βx−βy)/k(K is a wave number)Here, the calculation is carried out using a finite element method.
It is clear from FIG. 2 that the modal birefringence B, a measure of the polarization maintaining property, increases with an increase of the ratio (d2/d1). In addition, the ratio (d2/d1) equal to or greater than two can implement the birefringence equal to or greater than that of the conventional PANDA polarization-maintaining optical fiber (about 5×10−4 in PANDA). To increase (d2/d1), there is a method of increasing the diameter d2 or decreasing the diameter d1.
As for the polarization-maintaining optical fibers with such a structure, their prototypes and calculation examples are disclosed in the following two documents.
(1) A. Ortigosa-Blanch, J. C. Knight, W. J. Wadsworth, J. Arriaga, B. J. Mangan, T. A. Birks, P. St. Russell “Highly birefringent photonic crystal fibers” Optics Letters, Vol. 25, pp. 1325–1327 (2000); and
(2) S. B. Libori, J. Broeng, E. Knudsen, A. Bjarklev, “High-birefringent photonic crystal fiber” OFC 2001, TuM2, Anaheim (2001).
FIGS. 3 and 4 show cross-sectional structures of the polarization-maintaining optical fiber described in the foregoing documents: FIG. 3 shows a picture of an actually fabricated device; and FIG. 4 shows a structure drawn according to calculation values.
In the examples as shown in FIGS. 3 and 4, the diameters d2 of all the grating holes in the diffraction grating are less than the grating constant Λ. In addition, the diameter of the grating holes in a pair of opposed segments with respect to the core region differs from the diameter of the grating holes in another pair of opposed segments, thereby achieving the polarization maintaining characteristic.
In the polarization-maintaining optical fiber as shown in FIGS. 3 and 4, the modal birefringences B at a wavelength 1550 nm (calculation values) are 2.8×10−3 and 1.5×10−3, respectively.
However, when the diameter d1 is less than the grating constant as in the case of FIGS. 3 and 4, the optical confinement in the segments is weak, so that the light leaks from the core region to the segments, thereby the optical intensity distribution is distorted. In the worst case, the majority of the optical intensities might present outside of the core region. As a result, the fiber is susceptible to a bending loss, and becomes unusable as the fiber. Accordingly, it is impossible to set the diameter d1 below a certain value.
In addition, the example of FIG. 4 has a structure in which grating holes with a greater diameter deviate from the surrounding diffraction grating to the core region. However, it is difficult to fabricate such a polarization-maintaining optical fiber in practice.
As described above, the conventional polarization-maintaining optical fiber has a problem of having difficulty in implementing the following requirements at the same time: maintaining the polarization state of the signal light; carrying out long distance transmission; and fabricating with ease.
Furthermore, the conventional polarization-maintaining optical fiber allows the two orthogonal polarization modes to present within it. Thus, when the light travels a long distance through the fiber, slight crosstalk arises between the two polarization modes even if the polarization-maintaining optical fiber has the birefringence. Accordingly, it has a problem of inducing a perpendicular component at the output due to the polarization crosstalk, even if polarization state of the input light is set to one of the principal axes of the fiber. In fact, as for the PANDA fiber, the polarization crosstalk becomes a problem when the propagation distance exceeds 20 km.
Therefore an object of the present invention is to provide a polarization-maintaining optical fiber and absolutely single-polarization optical fiber capable of implementing the long distance transmission maintaining the polarization state of the optical signal.
Another object of the present invention to provide an absolutely single-polarization optical fiber enabling only one of the polarization modes to propagate through the fiber by providing a structure for absorbing the other of the polarization modes.